Knowing a voltage value is a common circuit requirement. For DC voltages, measuring the DC voltage value is straightforward as the value is constant. For AC voltages, the instantaneous value varies with time. However, it is still meaningful to know the average voltage over time, which can be calculated by taking the simple average of the voltage at each instant in the AC voltage waveform or, equivalently, the root mean squared (RMS) of the AC voltage. RMS is a statistical measure of the magnitude of a varying quantity. It is especially useful when the values are positive and negative, e.g. sinusoidal. The RMS value of a set of values, or a continuous-time waveform, is the square root of the arithmetic mean of the squares of the original values, or the square of the function that defines the continuous waveform.
Various conventional sensing circuits are used to measure an AC voltage, where the RMS value of the AC voltage may range from tens to hundreds of volts or more. FIG. 1 illustrates a conventional sensing circuit for measuring an AC voltage. The sensing circuit includes diodes D1 and D2, resistors R1, R2 and R3 for scaling high AC voltage down to low AC voltage, and capacitor C1. An AC voltage is provided to the diodes D1 and D2 and the purpose of the sensing circuit is to output a full-wave rectified voltage signal. FIG. 2 illustrates another conventional sensing circuit for measuring an AC voltage. The sensing circuit of FIG. 2 is similar to the circuit of FIG. 1 with the addition of an amplifier to reduce load impact. FIG. 3 illustrates yet another conventional sensing circuit for measuring an AC voltage. The sensing circuit of FIG. 3 includes diodes D1, D2, D3 and D4, resistors R1, R2, R3 and R4, capacitors C1 and C2, and an amplifier.
FIG. 4 illustrates exemplary voltage waveforms corresponding to the conventional sensing circuits. The top waveforms 2 and 4 show an exemplary AC voltage signal Vin. The middle waveforms 6 and 8 show rectified waveforms |Vin| of the input AC voltage signal. The bottom waveforms 10 and 12 show squared waveforms |Vin|2 of the rectified waveforms 6 and 8, respectively. Waveforms 2, 6 and 10 correspond to intermediate and heavy load conditions. Waveforms 4, 8 and 12 correspond to light load conditions. At intermediate and heavy load conditions, full-wave rectification is fully feasible, as shown in waveforms 6 and 10. In the case of digital measurement, the RMS voltage is determined by sampling the rectified waveform 6 according to a sampling rate, such as sampling points 14, 16, 18, 20, 22 and 24, by an analog-to-digital converter, which can be an ASIC chip or can be embedded in a digital signal controller (DSC), a digital signal processor (DSP) or a microcontroller (MCU), and calculating the square of the sampled voltage values and the RMS as the square root of the average of the squared voltage values over a period of time, such as a half-cycle T/2. However, at light load conditions, the rectified waveform |Vin| is distorted, as shown in waveform 8 where the voltage does not transit to zero. In fact, the voltage remains significantly above zero volts. The distorted rectified waveform |Vin| results in a distorted squared waveform |Vin|2, as shown in waveform 12. In order to measure a correct RMS voltage value using the sampling method, the rectified voltage waveform 8 should ideally be a full-wave rectified waveform similar to that of waveform 6. However, current methods of determining the RMS voltage assume full-wave rectified signals, even under light load conditions, and utilize sample values regardless of the distortion of the rectified and squared voltage waveforms.
A cause of the distorted rectified voltage waveform is the non-ideal nature of the diodes in the sensing circuit. For intermediate and heavy load conditions, there is sufficient current through the diodes for them to function properly. However, for light load conditions, the current through the diodes is too small for the diodes to function properly, resulting in the distorted rectified waveform. Due to the distorted rectified waveform, and the resulting distorted squared waveform, the measurement accuracy of the AC voltage is no longer guaranteed to be precise under light load conditions. In conventional sensing circuits, the RMS voltage value calculated using the sampling method for light load conditions is greater than the actual RMS voltage value. Additionally, the functionality of the under voltage latch off, which is common in server or telecommunication power supplies and which stops power supplies from normal operation due to the RMS value of the AC voltage being lower than certain thresholds, becomes unreliable under light load conditions.